1. Field of the Invention
This invention relates to a method for aralkylation of benzene and alkylbenzenes (hereinafter totally referred to as "alkylbenzenes"). More particularly, the invention relates to a method for aralkylation in which, when an olefin or a mixture of olefins and alkylbenzenes are subjected to addition reaction, only the aromatic olefins having double bonds that are conjugated with benzene rings (hereinafter referred to as "styrenes") are selectively aralkylated.
2. Description of the Prior Art
The addition of several olefins to alkylbenzenes is well known as one of Friedel-Crafts reactions since it is very important in view of chemical industry. The addition reactions are defined such that the reaction between alkylbenzenes and aliphatic double bond compounds (hereinafter referred to as "aliphatic olefins") is alkylation and the reaction between styrenes and alkylbenzenes is aralkylation. By the way, the term "olefins" includes both aliphatic olefins (including diolefins) and armomatic olefins (including styrenes). These definitions shall apply hereinafter.
The alkylated compounds are used as starting materials in chemical industry and the compound themselves are used as insulating oils, lubricating oils and so forth. Meanwhile, the aralkylated compounds are excellent in thermal stability, compatibility and electrical characteristics, so that they are industrially suitable as desirable aromatic synthetic oils for heat transfer medium, plasticizer, solvent, reaction medium and insulating oil. There are, however, few instances of chemical works to produce solely the aralkylated compounds because of the difficulty in preparation thereof.
The basic chemical structure of the aralkylated compound that is aimed to prepare in accordance with the present invention has a skeletal structure of diphenylmethane in which a pair of benzene rings are joined to one carbon atom. Accordingly, the products prepared through the method of the present invention are characterized in that they are noncondensed polycyclic aromatic synthetic oils which are different from the well known high boiling aromatic synthetic oils such as alkyl naphthalene and alkyl biphenyl.
There are known some methods to prepare the aralkylated products. In one method, alkylbenzenes are aralkylated with using .alpha.-halogenated alkylbenzenes in the presence of metal halide catalysts such as aluminum chloride. In another method, alkylbenzenes are aralkylated with styrenes in the presence of acid catalysts. In the former method, it is necessary to recover and treat the by-product of hydrogen halide and the economical acquisition of aralkylating agents is difficult. However, the latter method for aralkylation with the use of styrenes in the presence of acid catalyst is desirable because it is free from such the disadvantages.
As the methods for aralkylation with styrenes that have already been disclosed, are the method to use sulfuric acid catalyst described in British Pat. No. 977,322 and the method to use solid acid catalyst described in British Pat. No. 896,864. However, the present inventors have confirmed the fact that, in both the above methods, the addition reaction of not only styrenes but also aliphatic olefins are caused to proceed. Therefore, when the prior art method is employed for the raw material containing both styrenes and aliphatic olefins, a mixture of aralkylation product and alkylation product is obtained because of the lack of selectivity to styrenes or aliphatic olefins. In order to prepare the aralkylation product solely, it is necessary to use previously isolated styrenes as raw materials and the use of such the expensive materials is disadvantageous in view of the industrial production.